Synthetic quartz glass preform and device for the production thereof

ABSTRACT

A synthetic quartz glass preform is produced by flame hydrolysis with subsequent cooling and is suitable for the application of high-energy DUV radiation in the wave length range under 250 nm. The preform has a core area which contains ≧1150 ppm OH, a strain double refraction of ≦5 nm/cm and a resistance to high-energy DUV radiation as a result of a transmission reduction of ΔT≦0.1%/cm thickness. The quartz glass has been exposed to the following radiation: wavelength λ 1 =248 nm, laser shot frequency≧300 Hz, laser shot value≧10 9  and lumination≦10 mJ/cm 2 , and wavelength λ 2 =193 nm, laser shot frequency≧300 Hz, laser shot value≧10 9  and lumination&lt;5 mJ/cm 2 . Apparatus for producing the preform comprises a horizontally positioned muffle with two different sized openings facing each other. The larger of the openings is for removing the preform, the smaller opening being for introducing a burner. The internal chamber of the muffle narrows from the larger opening to the smaller opening.

REFERENCE TO RELATED APPLICATION

[0001] This is a divisional application of Ser. No. 09/381,490, filedSep. 15, 1999.

BACKGROUND OF THE INVENTION

[0002] The invention relates to a synthetic quartz glass preform and adevice for producing the same.

[0003] In accordance with developments in the semiconductor industry,and utilization of the products from the semiconductor industry invarious fields of application, as well as due to independentdevelopments, in particular in the special fields of materialsmanagement and medicine, light sources with very high energy densitiesfind application. Particularly, these are excimer laser with operationwavelengths of 248 nm and 193 nm. The optical components used therebyfor imaging and directing the radiation, as well as the photomasks whichexclusively consist of synthetic quartz glass or calcium fluoride haveto satisfy the required optical quality and must not lose the same incontinuous operation. The most important high-quality features of theoptical components and the most difficult ones to be set, are opticalhomogeneity and stability with respect to excimer laser irradiation inthe deep ultra-violet light (DUV). Therefore, there was no lack oftrials in the past to obtain such high-quality features permanently andreproducibly.

[0004] Hence, there is known a method for producing a homogeneous,striae free body of quartz glass from DE 42 04 406 A1, in which arod-shaped initial body is twisted, multiply thermally remodeled in amould of suitable foreign material and twisted again. In the EP 0 673888 A1 this method is modified under avoidance of any contact with aforeign material in such a manner that a quartz glass body subsequentlyproduced according to the method is optically homogeneous in threedirections and additionally is stable with respect to excimer laserradiation. However, EP 0 673 888 A1 does not teach to which degree thisstability is achieved. Additionally, the method is considerably time andcost consuming.

[0005] Synthetic quartz glass is characterized by having very goodtransmission in the deep range of ultra-violet light (DUV). When it isexposed to high energy short-wave radiation as, for example, provided byexcimer lasers at 248 nm and 193 nm, photochemical reactions willresult, which will lead to the formation of paramagnetic defects, thelatter being responsible for the formation of absorption bands and thedevelopment of luminescence. The power of these photochemical reactionsdepends on intrinsic defects in the form of binding anomalies. Thephotochemical reactions are also intensified by contaminants in anetwork as given, for example, by atoms of transition metals andchlorine. Parallel to these photochemical reactions which impair theoptical properties of the quartz glass, annealing processes take placefor which an OH content and a content of free hydrogen in the quartzglass is of importance.

[0006] From the subsequently discussed prior art it is known todesensitize synthetic quartz glass to high-energy radiation in the DUVby the following measures carried out individually or in combination:introducing molecular hydrogen into the quartz glass bulk, usingparticularly pure starting raw material, using chlorine free startingraw material, and doping the quartz glass with fluorine and others. TheEP 0 483 752 A1 ( U.S. Pat. No. 5,410,428) reference relates to asynthetic silica glass with a content of molecular hydrogen of at least5·10¹⁶ molecules/cm³ which is manufactured by a process wherein a quartzglass body is exposed to a hydrogen atmosphere in a furnace at a hightemperature and a high pressure for a defined time, until a desiredhydrogen concentration has been established in its interior;subsequently the silica glass body is definedly cooled down to ambienttemperature. This silica glass is known as being very stable againsthigh-energy radiation in the DUV, although it has only been exposed to2·10⁶ laser shots. It is disadvantageous that an aftertreatment of thesilica glass is necessary including extensive safety measures requiredthereto. Furthermore, the produced silica glass bodies exhibiting thedesired properties may not be of very large volume.

[0007] The EP 0 525 984 A1 reference describes a method for producingquartz glass which is adapted to be exposed to an excimer laserirradiation. However, the resistance property of the same is onlydisclosed up to a laser shot rate of about 10⁶ at an energy density of200 mJ/cm², a shot frequency of 100 Hz and a wavelength of λ=193 nm. Themethod does not function without a specific homogenizing step whichrenders it expensive.

[0008] The patent specification EP 0 737 654 A1 relates to a syntheticquartz glass with a content of molecular hydrogen of at least 10¹⁸molecules/cm³ and a low OH content of a maximum of 50 ppm, which at atemperature of maximally 500° C. and under a high pressure is enrichedwith H₂. The stability is specified with 1.3·10⁷ laser shots at anenergy density of 350 mJ/cm², a shot frequency of 400 Hz and awavelength of 248 nm, Also in this case, a subsequent treatment of thequartz glass is required, to which end a chlorine free raw material canbe used.

[0009] In U.S. Pat. No. 5,364,433 a synthetic quartz glass suited forproduction of DUV-stepper lenses and a method for producing the same isdisclosed. The quartz glass exhibits an OH content of 10-100 ppm, achlorine content of maximally 200 ppm, a molecular hydrogen content of<10¹⁶ molecules/cm³, a refractive index homogeneity of >5·10⁻⁶ and astrain of >5 nm/cm. The stability of this quartz glass against excimerlaser irradiation at a low absorption is only disclosed up to a low 10shot rate of 0.8·10⁶ (energy density 200 mJ/cm², shot frequency 100 Hz,λ=193 nm). The comparatively low stability is explained in that adehydration step provided for in the manufacturing process leads to anincrease of the Cl content which, in turn, reduces the DUV stability. Anadditionally provided homogenizing step renders the method moreexpensive.

[0010] A substrate plate for photomasks which shows a H₂ content between10¹⁷ and 10⁹ molecules/cm³ is disclosed in EP 0 636 586 A1. Thissolution is little or not at all suited for the production of imagingoptical members in the DUV range, which are subject to considerablyhigher requirements with regard to the transmission and the opticalhomogeneity than photomasks.

[0011] U.S. Pat. No. 5,086,352 discloses optical components made ofsynthetic quartz glass which can be employed in DUV excimer laserirradiation and a method for producing the same. The optical componentsexhibit an OH concentration of at least 100 ppm and a doped hydrogenconcentration of at least 5·10¹⁶ molecules/cm³ (and an amount of 1·10²⁰molecules/cm³ released at degassing, respectively,) and are free fromstratification in at least one direction. 50 ppb are given for thechemical purity of the component in the most pretentious case for Na, Kand LI, and 10 ppb for Mg, Ca, Ti, Cr, Fe, Ni, and Cu. The preforms ofthe optical components are characterized in that there is nostratification parallel to the incident light, that the OH concentrationrises from a central minimum to a maximum without a point of inflection,that in the range between rninimum and maximum the refractive indexinhomogeneity is 2·10³¹ ⁶ or lower, and in that there exists a hydrogendoping. Such a preform shows profiles of the OH concentration, of the Clconcentration and of a fictitious temperature which are to be adjustedfor obtaining a high refractive index of homogeneity. The method forproducing the optical components comprises, in each case, steps forremoving stratifications and for doping with hydrogen which renders theentire production process complicated and expensive. Moreover, thestability is only given up to a comparatively low laser shot rate of 10⁷(energy densities: 400 and 100 mJ/cm², respectively, shot frequencies:100 Hz, λ: 248 and 193 mn, respectively,).

[0012] U.S. Pat. No. 5,325,230 is based on U.S. Pat. No. 5,086,352 andadditionally requires for the optical component of synthetic quartzglass an absence of oxygen defects and a strain birefringence, which hasto be <5 nm/cm. The OH concentration distribution is axiallysymmetrical. Also in this case, stratifications have to be removed anddoping with hydrogen has to be carried out in the course ofmanufacturing the optical components. Considerable and expensive effortshave to be made to obtain a high purity of the quartz glass, which alsofinds expression in that special measures have to be taken for storingthe basic materials.

[0013] In the EP 0 747 327 A1 reference, a method for the heat treatmentand consolidation of a quartz glass preform is described whereby areduction of the laser induced defects in the quartz glass is assertedto be obtained. There is nothing reported of the refractive indexhomogeneity, of the form and mass of the bodies to be produced, of afeasible application of the produced quartz glass under extremeconditions. The represented increases in absorption at 248 nm and 193nm, respectively, are only acceptable up to few million shots.

[0014] EP 0 622 340 A1 discloses an improved method for producing a bodyof synthetic silica glass. A burner comprising at least five nozzles issupplied with fuel gas in such a manner that the produced syntheticsilica glass shows an OH content optimized compared to the H₂ content.There is nothing reported with respect to the DUV stability and therefractive index homogeneity. For obtaining OH contents above 1150 ppm,this procedure is unstable with respect to the attainable growthbehavior.

[0015] In EP 0720 969 A1, a quartz glass, an optical componentcontaining this quartz glass and a manufacturing process for the quartzglass are described. For the production of the preforms a downwarddirected burner is employed. The stability of the quartz glass withrespect to the excimer laser irradiation lies at a comparatively lowshot rate of about 10⁶. A Cl content of 10 ppm is achieved by anextremely low uneconomical raw material feed of 70 g/min·cm² via thecentral nozzle of the burner. The OH concentration of the quartz glasssubstantially lies at only 900 ppm.

[0016] In EP 0 720 970 A1 there is described a quartz glass forphotolithographic applications, an optical component containing saidquartz glass, a photolithographic device containing said component, anda method for producing the quartz glass. There are conditions disclosedfor producing a preform which can also be utilized in the DUV. However,the stability of the quartz glass against excimer laser irradiation isonly represented up to 10⁶ shots. The quartz glass is subjected to anF-doping which ensures, as known, low dispersion losses and has afavorable effect on the DUV stability. However, a high opticalhomogeneity of the melted quartz glass will not be attainable owing tothe F-doping. In the course of SiO₂ deposit on places with the highesttemperature, there also develop the highest OH concentration and thehighest F concentration. Hence, an error is introduced which increasesthe gradient of the refractive index curve.

[0017] Finally, EP 0 735 006 describes a method for producing quartzglass in which the growth process of the quartz glass producedsynthetically takes place in an upright direction. The process iscontrolled in such a manner that the stratification is adapted to takeplace only vertically to the growth direction of the preform.

SUMMARY OF THE INVENTION

[0018] By virtue of the present invention the disadvantages of the priorsynthetic quartz glasses are obviated which, up to now, did not permitthe utilization of the same in extreme applications in the DUV.Therefore, it is an object of the present invention to produce, underuse of a flame hydrolysis technique, a synthetic quartz glass whichmeets highest requirements concerning stability with respect to excimerlaser irradiation in the DUV at a high energy density and concerningoptical homogeneity. It is a further object of present invention toprovide a device which is particularly suited in the manufacture of thequartz glass and which renders the output of the manufacturing processmaximal.

[0019] According to the present invention, the objects are realized bythe characteristic features described herein. It is also feasible toemploy radiation of other wavelengths, provided that the same lies under250 nm. The excitation conditions can be varied; for example, atransmission reduction of ΔT<0.05%, is obtained at a laser shotfrequency (frequency) of >400 Hz, a laser shot rate (shot number) of>10⁸, and an energy density<25 mJ/cm² with respect to the wavelengthλ₁=248 nm. The transmission reduction corresponds to damage behavior forvalues stated herein. Hence, it lies within the scope of the invention.It can be generally stated that a varied reduction of the internaltransmission takes place at a radiation variation, but an unchangeddamage behavior. Damage behavior is to be understood by someone skilledin the art as a long-term damage, for example, a transmission variationof synthetic quartz glass under the effect of an excimer laserirradiation.

[0020] A core area of the preform extends over at least 50 to 90% of thepreform diameter, which can amount to up to 18 cm and more. It showsneither an axial stratification nor a stratification at right angles toits direction of growth; its entire volume is free from stratifications.The growth range of the drum-shaped preform has an at least almost flatpart close to the center, which substantially conforms to the core, anda peripheral part with a parabolic face which passes over into acylindrical surface of the drum-shaped preform. The cross-sectional areaof the preform which can be utilized for different purposes and in whichthe quality of the synthetic quartz meets the respective requirements isdifferent. Thus, for example, it is sufficient when used in illuminationsystems for excimer laser, that the synthetic quartz glass has highstability and transmission at an adequate homogeneity. In this case, 70%to 90% of the inner cross-section of the preform can be utilized. Whenprojection elements for directing high-energy laser irradiation are madefrom the preform, then under the same conditions a limitation to theinner 50 to 70% of the cross-section of the preform is necessary.Thereby, it is essential that across the inner cross-section of thepreform not only high stability and transmission exist, but also highhomogeneity; this means, however, that the OH content of the preform isconstant to ±10 ppm over this inner cross-section. Advantageously, theOH content of the core area of the preform amounts to at least 1250 ppmat a tolerance of ±10 ppm. The Cl content of the same does not exceed 20ppm and preferably is 5 to 15 ppm. The H₂ content of the core area ofthe preform advantageously amounts to >1·10¹⁸ molecules/cm³. A preformhaving the abovementioned parameters is, to a high degree, stableagainst high energy DUV irradiation, shows a high refractive indexstability and is excellent for the production of optical members such asDUV stepper-lenses, directing members for laser beams, photomasks etc.At least over a part of the core area, the preform advantageouslyexhibits a refractive index homogeneity of <0.5·10⁻⁶. Thereby, traces ofcontaminating elements (e.g. Cr, Co, Fe, Ni, Cu, V, Zn, Al, Li, K, Na)can be contained in the preform up to 500 ppb. The preform does notrequire any additional doping with H₂, F and others, to render itserviceable for tasks in DUV excimer laser irradiation. Also asubsequent treatment of the synthetic quartz glass in a reducingatmosphere is not required. If necessary, it is advantageous to cutoptical members out of the material of the core area.

[0021] A device for producing the preform comprises a substantiallyhorizontal muffle with two differently sized arranged openings opposingeach other, the larger of which is adapted for inserting the preform andthe smaller one for inserting a burner, and an internal chamber whichnarrows from the larger opening to the smaller opening. The burner isprovided with nozzles which are coaxially arranged to each other and tothe burner axis, the centrally arranged nozzle discharges the basicmaterial, for example, SiCl₄ and O₂ and the external nozzles the fuelgas, for example, H₂ and O₂, parallel to one another and to the burneraxis. The narrowing substantially is a gradual one. Unlike similar priordevices, the muffle has neither an opening nor a bulge on its top-side.The overall length of the muffle is at least twice the size of thediameter of the vitreous preform. The almost planar leading face of thelatter is preferably arranged in the center of the internal chamber ofthe muffle. The muffle is preferably embodied in three layers in orderto ensure a sufficient and constant internal temperature as well as alow heat emission. It is advantageous, when the distance of thesubstantially rotation-symmetrical preform surface relative to theinternal limiting face is 5 to 100 mm depending on the flow conditionsfor the waste gas. Furthermore, it is advantageous, when the distance ofthe burner to the preform is 135 to 350 mm in dependence on the geometryof the burner nozzles and the flow of the fuel gases volume. As to thesmaller opening, in which the burner is arranged for free movement, adiameter of 50 to 100 mm is to be recommended.

[0022] Due to the internal geometry of the muffle and the operation ofthe burner, the device of the present invention ensures that the preformis definedly distributed by the fuel gas as well as that preforms ofprincipally optional lengths can be the melted on. No subsequenttreatment (twisting, doping) is required for the preform. A modificationof geometry in order to adapt the preform to the intended applicationcan be combined with a heating of the preform. In spite of extremeprocess control the device permits melting preform masses of 50 kg andmore, in a normal melting process, which are optically homogeneous andstable in the DUV against high-energy laser beams.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention will be explained hereinafter in more detail byvirtue of the schematic drawings. There is shown in:

[0024]FIG. 1 a device for melting on a preform,

[0025]FIG. 2 a diagram, in which internal transmission of a quartz glassblock, cut out of a preform, is plotted as a function of laser pulserate at a wavelength of λ₁=248 nm,

[0026]FIG. 3 a diagram corresponding to that of FIG. 2 for λ₂=193 nm,

[0027]FIG. 4 a top view of a preform, and

[0028]FIGS. 5a-5 d four graphs obtained from a laser inducedfluorescence (LIF) at a transmission with laser light of the wavelengthsλ₁=248 nm and λ₂=193 nm, from the OH content and the strain-inducedbirefringence.

DETAILED DESCRIPTION

[0029] In FIG. 1, a horizontally arranged muffle 10, which effects lowheat losses caused by radiation, is represented by three shells 11, 12,13, wherein the shell 11 is a high porosity insulating material, forexample, a ceramic fiber material, shell 12 is a fire-resisting concreteor chamotte, and shell 13 is a high temperature resistant material, forexample, Al₂O₃ or SiC. The muffle 10 has an internal chamber 14 with asmaller opening 15 for inserting a burner 16 and a larger opening 17 viawhich a preform 18 to be melted on protrudes into the muffle 10, thegeometrical axis of said preform coincides with an axis of rotation X-X.At least a portion of the muffle 10 which envelopes the preform 18 is atleast approximately symmetrical about the axis X-X, too. There is aspace a between a parabolic face 19 of the preform 18 and a limitingsurface 20 of the internal chamber 14. The space a advantageously is notlarger than 50 mm and not smaller than 15 mm in order to eliminatedeposits on the limiting surface 20 by the material to be melted. In theinternal chamber 14, the preform 18 is provided with a cap 21 having asubstantially planar leading face of which a plateau 22 lies in thecenter of the muffle 10 and is at right angles to the axis X-X. Theparabolic lateral face 19 of the preform 18 is a side of the cap 21. Viathe opening 15, which in the present embodiment has a diameter of 60 mm,the burner 16 is inserted into that portion of the muffle 10 whichdeviates from the axial symmetry, in such a manner that its axis Y-Y isslightly inclined relative to the axis of rotation X-X and intersectsthe plateau 22 below the intersection point of the axis of rotation X-Xand the plateau 22. The burner 16 is provided with a plurality ofnozzles, not shown in detail, which are in parallel to each other. Acentrally arranged nozzle discharges 410 g/min·cm² SiCl₄ and nozzlesarranged peripherally to the former, discharge 14.5 m³/h O₂ as well as 7m³/h O₂ so that a growth rate of 8 mm/h results. The burner 16 isadjustable within the opening 15. The torch 23 of the burner 16 isdirected towards the plateau 22.

[0030] In a method for producing the preform 18 from synthetic quartzglass, which principally is taught, for example, in DE 42 03 287 C2,SiO₂ particles are formed from SiCl₄ by means of an H₂/O₂ flame andimmediately vitreously melted on at temperatures of over 2000° C. toyield the drum-shaped, glassy preform 18. The preform 18 has an axiallysymmetrical refractive index profile. The preform 18 is taken from thearrangement after completion of the melting process and is subjected toa conventional cooling process in order to reduce internal strains to <5nm/cm strain birefringence. The preform 18 does not show anystratifications. By virtue of the arrangement described hereinbefore,the drum-shaped preform 18 is produced, the synthetic quartz glass ofwhich exhibits the abovementioned parameters concerning the OH contentand the Cl content, as well as the internal transmission and theoutstanding low decrease of transmission under the specified radiationconditions as well as a high optical homogeneity. They are specified inthe following figures.

[0031] In the Cartesian coordinate system of FIG. 2 laser pulse numbers100 up to 1100, multiplied by 10⁶ are plotted along the x-coordinate andthe internal transmission in % is plotted along the y-coordinate at athickness of the glass layer of 10 mm. A curve 1 represents the internaltransmission T_(i) for laser light of the wavelength 248 nm for thequartz glass, which is very high at 99.84% and is constant up to 700·10⁶laser pulses. Only then it slightly slopes, namely by 0.02%, up to1100·10⁶ laser pulses. Accordingly, the decrease in transmission ΔT liesat 900·10⁶ laser pulses far below the value of 0.1% mentioned above. Thefurther conditions are: laser frequency=300 Hz, energy density=10mJ/cm².

[0032] As to the Cartesian coordinate system of FIG. 3 the same measuresare valid as in FIG. 2. The quartz glass of 10 mm thickness is exposedto a laser irradiation of the following conditions: λ₂=193 mn, laserfrequency=300 Hz, energy density=1.5 mJ/cm². Curve 2 represents theinternal transmission of the quartz glass, which is constant up to300·10⁶ laser pulses, and which decreases by 0.05% between 300·10⁶ and500·10⁶ laser pulses, and between 500·10⁶ laser pulses and 700·10⁶ laserpulses by 0.04%, and then remains constant up to 1100·10⁶ laser pulses.Also in this case, the condition for the transmission decreaseΔT<0.1%/cm is maintained.

[0033] In FIG. 4 a section of a plan view of a preform 18 is representedwith a radius r=6 cm. The radius vector r is the x-coordinate in thefollowing FIG. 5. Thereby, the excitation conditions for the laserirradiation as mentioned in FIG. 2 and 3 are also valid.

[0034] In FIG. 5a the LIF values for the wavelength λ₁, are plotted in acurve 3 as a function of the radius by centimeters. In order todetermine the LIF values, stability of optical components under theeffect of laser radiation, as published in W. Triebel et al. in theJournal Technisches Messen, vol. 63 (1996), number 7/8, pp. 291-295 havebeen utilized. The state of an unchanged luminescence was obtained after2000 laser shots, in measuring an unchanged luminescence at a wavelengthof 650 nm. The LIF values of 0.7 to 2.5 determined via the radius lie atabout {fraction (1/10)} of the LIF values of the relevant products ofthe prior art. The same is valid for the LIF values of the wavelength 2,which in FIG. 5b are plotted in a curve 6, centimeter by centimeter ofthe radius and, from the center up to the edge of the preform 18, rangefrom 0.55 to 1.8.

[0035] In FIG. 5c the OH content detected via the radius is plottedcentimeter by centimeter in a curve 4. Thereby it becomes obvious thatthe OH content in a core area of 4 cm exceeds by far a minimum value of1150 ppm and that the minimum value, even at the periphery of thepreform 18, is set at 1180 ppm.

[0036] In FIG. 5d the values of the strain-induced birefringence (SDB)measured as a function of the radius are plotted in a curve 5. There itbecomes obvious that the values, at least in the core area up to 4 cm,fall far below the limiting value of 5 nm/cm layer thickness for thestrain-induced birefringence and that in the edge portion (r=5 to 6 cm)of the preform 18 this limiting value is at least almost maintained.

[0037] All features disclosed in the specification and in the drawingsare substantial for the invention both, individually and in anycombination with one another.

What we claim is:
 1. Apparatus for producing a synthetic quartz glasspreform by flame hydrolysis while being rotated about an axis ofrotation thereof and with subsequent cooling and which is adapted forapplication of high-energy DUV-radiation in the wavelength range under250 nm, the preform having a core with an OH-content of ≧1150 ppm, astrain birefringence of ≦5 nm/cm, a H₂-content of ≧1·10¹⁸ molecules/cm³,a CI-content of ≦20 ppm, an amount of contaminating trace elements Cr,Co, Fe, Ni, Cu, V, Zn, Al, Li, K, Na up to 500 ppb and which issubstantially free of stratifications, and the stability of whichtowards high-energy DUV-radiation is given by a transmissions reductionof ΔT≦0.1%/cm thickness, when the synthetic quartz glass is subjected tothe following radiation, wavelength λ₁=248 nm, laser shot frequency>300Hz, laser shot rate≧10⁹, and energy density≦10 mJ/cm², as well aswavelength λ₂=193 nm, laser shot frequency≧300 Hz, laser shot rate≧10⁹,and energy density≦5 mJ/cm², the apparatus comprising a horizontallyarranged muffle, the muffle comprising an internal chamber, a largeropening and a smaller opening communicating with the chamber on oppositesides of the muffle, the larger opening being adapted for removing thepreform and the smaller one for receiving a burner, the chambersubstantially gradually narrowing from the larger opening to the smalleropening, the chamber having a portion adapted to envelope said preform,at least said portion being at least approximately rotation-symmetricalto the axis of rotation of said preform.
 2. Apparatus as claimed inclaim 1, wherein the entire length of the muffle is at least equal todouble the diameter of the preform.
 3. Apparatus as claimed in claim 1,wherein the chamber has a center adapted to receive a leading face ofthe preform.
 4. Apparatus as claimed in claim 1, wherein the muffle iscomprised of three superimposed shells.
 5. Apparatus as claimed in claim1, wherein the chamber is so dimensioned that a distance from a surfaceof the preform to a nearest surface of the chamber is 5 to 100 mm. 6.Apparatus as claimed in claim 1, further comprising a burner and whereinthe distance of the burner to a position in the chamber where theleading face of the preform is received is 135 to 350 mm.
 7. Device asclaimed in claim 11, wherein the smaller opening has a diameter of 50 to100 mm.